I'd suggest you need at least 3 pots, 2 feet deep by 12 inches wide minimum - any climber contained in a pot won't reach its full potential and cover a large area such as you've mentioned, so placing 3 pots with climbers would probably be necessary to achieve better cover. You're also restricted as to what you can successfully use as its so shady. Suggestions for plants include Hydrangea anomala; Schizophragma integrifolia; Garrya elliptica (not a climber, but a wall shrub); Hedera helix varieties. All of these prefer shade.

Added to GoYpedia Shade Container Plants..

Many thanks for your advice 'Bamboo'

There also one or two clematis that prefer a position out of full sun. 'Guernsey Cream' and 'Nelly Moser' would both be quite happy on your garage wall

Good point, Andrew - Clematis Hagley Hybrid would be okay too.

what about virginia creeper ? it can be trained into a few pots and looks lovley hanging down round the edge .